1979 75 hp piston problem evinrude

[gary250]

lost compression in top clyinder pulled head and found a chip down to the ring aprox 1/2 in -1/4 in the top of the piston near exhaust port the peice must have went strait out the port because there is no damage my question is what would cause this to happen, the clyinders measure out to spec.

 

[Hooty]

Re: 1979 75 hp piston problem evinrude

My guess is something snagged the exhaust port. Probably a ring. You won't know for sure 'till you get into it though.<br /><br />c/6<br />Hooty

 

[gary250]

Re: 1979 75 hp piston problem evinrude

I can see that the ring is bent would carbon cause this to snag and break off the piston

 

[Dhadley]

Re: 1979 75 hp piston problem evinrude

The corner of the piston being knocked off at the exhaust port is usually caused by shock waves created by pre-ignition/detonation. Carbon build up on the piston dome could be the cause. So could an out of time ignition (too much advance, duble firing etc.) Coking usually causes the rings to stick in the ring groove and as the piston loses support the ring end is sheared off. Not necissarily next to a port. <br /><br />The corner of the piston being knocked off usually happens near the exhaust port as that is the hottest area on the piston dome.<br /><br />There is a reason for the piston damage. Something caused it. The corner did not "fall" off by itself. The cause must be identified and corrected so this does not happen again.<br /><br />Good luck!